Giant light enhancement in atomic clusters
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Giant Light Enhancement in Atomic Clusters O. N. Gadomskya*, I. V. Gadomskayaa, and K. K. Altuninb** a
Ulyanovsk State University, Ulyanovsk, 432700 Russia b Ulyanovsk State Pedagogical University, Ulyanovsk, 432700 Russia *email: [email protected] **email: [email protected] Received February 13, 2009
Abstract—We show that the polarizing effect of the atoms in an atomic cluster can lead to full compensation of the radiative damping of excited atomic states, a change in the sign of the dispersion of the atomic polar izability, and giant light enhancement by the atomic cluster. PACS numbers: 61.46.w, 61.80.Az DOI: 10.1134/S1063776109070036
[4, 5], in this paper, we focus our attention not on the dimensional resonances but on the factors that can affect significantly the natural width of atomic levels in a cluster.
1. INTRODUCTION The detection of intense photoluminescence of gold clusters in the wavelength range from the ultravi olet to the near infrared, depending on the number of gold atoms in the cluster, was reported in the experi mental works [1, 2]. The quantum yield of lumines cence was very high: from 70% for the smallest Au5 clusters to 10% for the largest Au31 clusters. The induction of polarizing internal fields at the locations of the cluster atoms is responsible for this effect. In the effect of surfaceenhanced Raman scattering [3], it is well known that if molecules like pyridine ones are adsorbed or located near a metal (e.g., silver) sur face, then the Raman scattering cross section for many of their vibrational modes increases by a factor of 102 ⎯106 compared to that for free molecules. The distinctive feature of the light enhancement effect presented here is that the inversion, i.e., the dif ference of the probabilities of the cluster atoms being detected in the excited and ground states, is almost equal to a stationary value of w0 = 1/2. The enhance ment effect is attributable not to the superradiation of the atomic cluster but to the coherent effect of polar izing fields inside the cluster, which changes the pro file of the natural width of the quantum transition line. In [4, 5], it was shown that linear optical dimen sional resonances whose frequencies differ signifi cantly from the transition frequency in the spectrum of the cluster atoms are formed in atomic clusters at an equilibrium inversion of w0 = ⎯1. The optical proper ties of nonlinear resonances, when the inversion w dif fers from ⎯1, were considered in [6]. The dimensional resonances were shown to be of great importance in optical nearfield microscopy [7] and in controlling the motion of atoms by a laser field [8]. In contrast to
2. QUASISTATIONARY EXCITATION OF LIGHT IN AN ATOMIC CLUSTER Consider a diatomic cluster by placing the coordi nate origin at the location of one of the atoms, for example, atom 1. The cluster axis R12 that connects the atomic centers is directed along the y axis. The wave vector k0 of the external wave is perpendicular to the axis R12, while the polarization vector
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